1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a printer or the like, which adopts an electrophotographic method or an electrostatic recording method.
2. Description of the Related Art
A description will now be provided of a color laser printer as a conventional image forming apparatus with reference to FIG. 6.
FIG. 6 is a cross-sectional view of a conventional color laser printer. In FIG. 6, reference numeral 104a represents an image bearing member. A charging roller 104b serves as a primary charger in pressure contact with the image bearing member 104a. By applying a voltage to the charging roller 104b, the surface of the image bearing member 104a is uniformly charged before forming a latent image.
Exposure in the image bearing member 104a is performed by a scanner unit 110a which includes a laser diode. The laser diode emits light in accordance with an image signal, and projects a laser beam onto a polygonal mirror (not shown). The polygonal mirror is rotated at a high speed by a scanner motor (not shown) to reflect the laser beam. The laser beam reflected by the polygonal mirror selectively exposes the external circumferential surface of the image bearing member 104a after passing through a combined lens 110b and a reflecting mirror 110c. As a result, an electrostatic latent image is formed on the image bearing member 104a by the exposure of the laser beam.
The electrostatic latent image is developed by a rotating developing device 105 to provide a toner image of each color toner. The rotating developing device 105 includes developing units 105M, 105C, 105Y and 105K for a plurality of colors, i.e., magenta, cyan, yellow and black, respectively. The developing units 105M, 105C, 105Y and 105K for the four colors are disposed so as to be rotatable around a central axis 105e of the rotating developing device 105. The center of each of the developing units 105M, 105C, 105Y and 105K is rotatably linked with a gear disposed at the external circumference of a revolving gear to maintain its posture constant.
During image formation, the developing unit corresponding to the latent image, i.e., the magenta developing unit 105M in FIG. 6, stands still at a position facing the image bearing member 104a, and its developing sleeve 105b is positioned so as to face the surface of the image bearing member 104a with a small gap.
When a predetermined developing unit, i.e., the magenta developing unit 105M in FIG. 6, is rotatably moved to a developing position, a developing bias voltage is applied to the developing sleeve 105b by connecting the developing sleeve 105b to a high-voltage power supply of the main body of the printer. At the same time, the developing sleeve 105b is coupled with driving means from a driving source to perform predetermined rotation. During development, by applying the developing bias voltage to and rotatably driving the developing sleeve 105b, the latent image on the image bearing member 104a is developed to provide a visible toner image.
A sheet feeding unit 101 for feeding a transfer material 102 to a tranfer drum 103 includes a sheet feeding cassette 101a, for accommodating sheets of the transfer material 102, which is mounted in a base portion of the main body of the printer. During image formation, a sheet feeding roller 101b rotates in accordance with an image forming operation to individually separate sheets of the transfer material 102 from within the sheet feeding cassette 101a and to feed a separated sheet of the transfer material 102 to the transfer drum 103.
The transfer drum 103 is rotated at substantially the same speed as the circumferential speed of the image bearing member 104a (for example, 75.4 mm/sec) (hereinafter termed a "process speed") in order to wind the transfer material 102 fed from the sheet feeding unit 101 therearound and transfer the magenta toner image formed on the image bearing member 104a onto the transfer material 102 at a transfer nip.
The transfer drum 103 is configured by forming an elastic layer 103b, made of a sponge, rubber or the like, on the outer circumference of an aluminum cylinder 103a having a diameter of 180 mm, forming a resistive layer 103c on the outer circumference of the elastic layer 103b, and forming a dielectric layer 103d on the resistive layer 103c. A gripper 103f for gripping the leading edge of the fed transfer material 102 is provided at a predetermined position on the outer circumference of the transfer drum 103. An electrostatic attracting roller 103g is detachably provided so as to face the outer circumference of the transfer drum 103 and to press the transfer material 102 against the outer circumference of the transfer drum 103. By applying a voltage between the electrostatic attracting roller 103g and the transfer drum 103, charges are induced on the transfer material 102, which is a dielectric material, and the dielectric layer 103d of the transfer drum 103 to electrostatically attract the transfer material 102 onto the outer circumference of the transfer drum 103.
A cleaner 104d for cleaning toner particles remaining on the image bearing member 104a after transferring the toner image onto the transfer material 102 is disposed in the vicinity of the outer circumference of the image bearing member 104a at a portion downstream from the transfer portion.
A fixing unit 106 includes a rotatably driven pressing roller 106a and a fixing roller 106b for supplying the transfer material 102 with heat and pressure in a state of pressure contact with the pressing roller 106a. By passing the transfer material 102 peeled and conveyed from the transfer drum 103 while carrying the toner images of the respective colors through the fixing unit 106, the toner images of the respective colors are fixed.
In an image forming operation, the transfer material 102 within the sheet feeding cassette 101a is fed to the transfer drum 103 by the sheet feeding roller 101b. The transfer drum 103 grips the leading edge of the fed transfer material 102 with the gripper 103f and attracts the transfer material 102 on its circumferential surface.
On the other hand, a magenta image is exposed on the image bearing member 104a, whose surface has been uniformly charged by the charging roller 104b, by the scanner unit 110a, to form a magenta latent image on the outer circumference of the image bearing member 104a. The magenta developing unit 105M is driven simultaneously with the formation of the latent image. That is, the magenta latent image formed on the image bearing member 104a is developed by applying a developing bias voltage having the same polarity and substantially the same potential as the charging polarity of the image bearing member 104a so as to cause a magenta toner to adhere to the latent image to form a magenta toner image on the image bearing member 104a. Then, by applying a transfer voltage having a polarity inverse to the polarity of the magenta toner to the transfer drum 103, the magenta toner image on the image bearing member 104a is transferred onto the transfer material 102 on the transfer drum 103.
Upon completion of the transfer of the magenta toner image, the cyan developing unit 105C in the next step is rotated and positioned to a developing position facing the image bearing member 104a. Latent images for cyan, yellow and black toners are sequentially formed and developed, and obtained toner images are sequentially transferred in the same manner as in the case of the magenta image to form a full-color image on the transfer material 102.
By four rotations of the transfer drum 103 gripping and holding the transfer material 102, a full-color image comprising four colors can be obtained. That is, a full-color image is output in 180.pi..times.4/75.4=30 seconds.
The transfer material 102 after completion of transfer of toner images of four colors is separated from the tranfer drum 103 and is conveyed to the fixing unit 106. After fixing the full-color toner image by the fixing unit 106, the transfer material 102 is discharged onto a discharged-sheet tray 108 by a pair of discharging rollers 107.
In the above-described conventional color laser printer, however, both of a driving gear for the tranfer drum 103 and a driving gear for the image bearing member 104a are directly driven by a single driving motor, and the transfer drum 103 and the image bearing member 104a are made to be in pressure contact with each other at a transfer nip portion. Hence, the vibration of the motor within the closed-loop driving mechanism is amplified to strongly vibrate the image bearing member 104a and the transfer drum 103, resulting in disturbance in the latent image formed by the laser beam or a failure in image transfer.
Such problems also arise in image forming apparatuses in which a toner image on an image bearing member is directly transferred onto an intermediate transfer member and the toner image on the intermediate transfer member is then transferred onto a transfer material.